Varactors and capacitors are commonly used in circuits such as voltage controlled oscillators (VCOs), digitally controlled oscillators (DCOs), low noise amplifiers (LNAs), or tunable filters for tuning the oscillation or resonance frequency of these circuits. In DCOs, varactors usually operate at either the maximum or minimum capacitance (Cmax or Cmin). The capacitance difference between Cmax and Cmin values of the varactor determines the frequency step and resolution of the DCO.
The minimum value of the capacitance difference achievable with a single varactor is limited by the technology of the varactor, and is typically within the range of hundreds of attofarads (aF) for modern CMOS technologies. In some applications, the frequency deviation to the carrier during modulation requires frequency resolutions of below 1 kilohertz (kHz). This is smaller than 1 part-per-million (ppm) of oscillation frequency, which requires a capacitance difference of below 1 aF. It is therefore desirable to provide a varactor with improved resolution.
The minimum value of the capacitance achievable with a single capacitor is also limited by the technology of the capacitor, and is typically within the range of tens of femtofarads (fF) for modern CMOS technologies. In some applications, for instance for coarse oscillation frequency tuning of an oscillator, it would be desirable to have smaller capacitors for achieving higher coarse tuning frequency resolution.